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Why
Does A Thermos Keep Things Cold Longer Than It Keeps Things Hot?
John
Staughton
A thermos
appears to be more effective for keeping cold liquids chilled because the
difference in temperature between the ambient temperature and perceived “cold”
and “hot” liquids.
Heat
energy will transfer out of hot liquids more rapidly than it will transfer into
cold liquids.
Can you
remember back to lunchtime during your school days?
For me,
my lunch bag would contain a peanut butter and jelly sandwich, an apple, a bag
of chips and a room temperature Capri Sun.
I still
remember the envy I had for those kids who would pull out a thermos from their
bag and begin slurping up warm soup or drinking ice-cold juice.
The
apparent magic of a thermos has perplexed me for many years, but I have also
learned an interesting fact from various thermos-loving friends.
In short,
their thermoses are better at keeping cold liquids cold than they are at
keeping hot liquids hot. Why is that?
What is a
Thermos?
Before we
can understand its intricacies, let’s take a broad look at thermoses to have a
clearer idea of the subject.
A thermos
is any container that is able to keep liquids either hot or cold, thanks to a
double-walled design featuring a vacuum.
More
formally known as a vacuum flask, this invention has been around since the end
of the 19th century, when researcher Sir James Dewar stumbled upon it in the
course of his study of cryogenics.
Dewar
placed two brass flasks inside of one another, and joined them at the neck. The
air that became trapped between the two flasks in a thermos can be removed or
evacuated, creating a vacuum inside the space.
Without
any air to facilitate heat transfer, liquid placed in the inner flask will be
isolated from the exterior conditions, and will thus be able to retain its
temperature more effectively.
Although
this was a rudimentary version of a thermos, and only offered a partial vacuum,
it was a huge breakthrough.
Basically,
whether you put steaming hot soup or ice-cold water in the thermos, the vacuum
bubble surrounding the inner flask will keep the temperature stable for an
extended period of time.
The best
thermoses on the market claim to keep items cold or hot for two days, and up to
10 days if the liquid is iced!
While
thermos efficiency has certainly improved over the years, people still claim
that liquids cool off before they warm up, a claim that is directly related to
heat transfer.
Heat
Transfer in a Thermos
Energy is
created by the vibrational movement of atoms and molecules, and this energy can
be transferred.
When this
transfer of energy occurs as a result of a temperature difference, it is called
heat energy. In the case of differing temperatures, heat energy flows from a
hot object to a cold object.
As
mentioned above, a vacuum flask is able to prevent or minimize heat transfer
between the exterior environment and the liquid inside the thermos.
Why does
this matter? Because heat transfer is a key part of thermal engineering and
thermodynamics, and it occurs all across the universe!
Heat
transfer is how heat energy is transferred between objects, whether that is a
convection stove boiling a pot of water, the sun heating the planet or a
campfire warming a marshmallow!
When it
comes to the contents of a thermos and the exterior environment, heat transfer
is significantly delayed by the vacuum layer.
If it was
filled with hot soup, the heat energy from the interior flask would try to heat
up the colder gas molecules in the space between, which could then pass that
heat energy to the outer flask wall and the exterior environment temperature,
which would be colder than the hot soup.
However,
there is no gas in the vacuum space, so that transfer of heat energy does not
readily occur.
Similarly,
if the inner flask is filled with ice water, the warmth of the outer environment
will warm the flask, but there are no air molecules to transfer that heat
energy across to the inner flask wall, and thus the cold water stays cold!
So Why
Are Thermoses Better At Keeping Things Cold?
Concerning
heat transfer, the greater the difference in temperature between two objects,
the faster the energy will transfer between them, from hot to cold.
Our
perception of hot and cold is also an important thing to consider in this
question.
Consider
that room temperature is about 70 degrees Fahrenheit (21 Celsius); water
freezes at 32 degrees Fahrenheit (0 Celsius) and boils at 212 degrees
Fahrenheit (100 Celsius).
Thus, the
difference between room temperature and a thermos of an ice-cold drink is much
smaller than the difference between room temperature and a thermos of hot soup
off the stove.
Now, a
thermos is far from perfect, and some heat is gradually lost.
Every
time you open the thermos, for example, a significant amount of heat transfer
will occur (i.e., your coffee will cool or your water will warm).
However,
a larger amount of energy will be transferred between coffee and the cooler
air, due to the larger difference in temperature.
A smaller
amount of energy will transfer between the cold water and the warmer air, due
to their smaller difference in temperature.
This is
where human perception comes into play; we will be more aware of the
temperature change in the coffee than the temperature change in the water.
As a
result, popular opinion states that thermoses are better at keeping things cold
than keeping things warm, but by percentage of heat energy transferred,
thermoses are equally efficient at either end of the temperature spectrum.
A Final
Word
The
reason that thermoses seem more effective at keeping things chilled than warm
is our perception of the temperature change.
Hot
liquids (i.e., coffee and soup) are prepared considerably hotter than room
temperature, whereas a cold drink is much closer to room temperature.
As heat
transfer occurs, we are simply more aware of the difference in heat energy as
hot liquids change.
A good
way to avoid disappointment is to consume the contents of your thermos
immediately upon opening it for the very first time, before heat energy has a
chance to make its moves!
John Staughton is a traveling writer, editor, publisher and photographer who earned his English and Integrative Biology degrees from the University of Illinois. He is the co-founder of a literary journal, Sheriff Nottingham, and the Content Director for Stain’d Arts, an arts nonprofit based in Denver. On a perpetual journey towards the idea of home, he uses words to educate, inspire, uplift and evolve.
John Staughton is a traveling writer, editor, publisher and photographer who earned his English and Integrative Biology degrees from the University of Illinois. He is the co-founder of a literary journal, Sheriff Nottingham, and the Content Director for Stain’d Arts, an arts nonprofit based in Denver. On a perpetual journey towards the idea of home, he uses words to educate, inspire, uplift and evolve.
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